Silicon Mitigates Negative Impacts of Drought and UV-B Radiation in Plants

Due to climate change, plants are being more adversely affected by heatwaves, floods, droughts, and increased temperatures and UV radiation. This review focuses on enhanced UV-B radiation and drought, and mitigation of their adverse effects through silicon addition. Studies on UV-B stress and addition of silicon or silicon nanoparticles have been reported for crop plants including rice, wheat, and soybean. These have shown that addition of silicon to plants under UV-B radiation stress increases the contents of chlorophyll, soluble sugars, anthocyanins, flavonoids, and UV-absorbing and antioxidant compounds. Silicon also affects photosynthesis rate, proline content, metal toxicity, and lipid peroxidation. Drought is a stress factor that affects normal plant growth and development. It has been frequently reported that silicon can reduce stress caused by different abiotic factors, including drought. For example, under drought stress, silicon increases ascorbate peroxidase activity, total soluble sugars content, relative water content, and photosynthetic rate. Silicon also decreases peroxidase, catalase, and superoxide dismutase activities, and malondialdehyde content. The effects of silicon on drought and concurrently UV-B stressed plants has not yet been studied in detail, but initial studies show some stress mitigation by silicon.


Introduction
Plants are exposed to various biotic and abiotic stress factors. How abiotic factors such as drought and UV-B radiation impact plants are currently under more intense investigation due to the increasing threat of drought due to climate change. Water shortages result in lower yields of crop plants and limited crop growth, which, in turn, reduces food production. Ozone depletion due to atmospheric pollution also means that higher levels of UV-B radiation are reaching the Earth's surface. UV radiation is an essential factor for plant growth, but in excess, it can lead to oxidative damage to cells [1,2].
Silicon (Si) treatment of plants has been shown to mitigate some of the negative impacts of drought and enhanced UV-B radiation. In plants exposed to UV-B, Si can increase the activity of the photosynthetic apparatus, decrease the transpiration rate, increase the antioxidant capacity [3,4], and lower the concentrations of reactive oxygen species (ROS) [5] and protective phenol substances, in comparison to plants exposed to UV-B without Si treatment [4]. Similarly, during drought, Si can increase plant growth, relative water content, photosynthesis rate, and chlorophyll content, and affects other physiological responses [6]. This review presents current knowledge and studies about the effect of Si fertilization on mitigation of stress in plants caused by drought and/or excessed UV-B radiation, which often occur together. Though there are many drought and Si related review articles [7][8][9], the UV-B stress and Si and especially the joint effect of drought and UV-B stress and Si fertilization has not been studied in detail yet. ciency, actual photochemical quantum efficiency (φ PSII ), photochemical quenching coefficient (q P ), and photosynthetic electron transport rate [48,50]. In addition, the PetE, PetF, PsbP, PsbQ, PsbW, and Psb28 genes are related to photosynthesis, and their expression was down-regulated under water deficiency, but up-regulated by Si [48]. Si enhanced the net photosynthetic rates in tomato plants and wheat under drought stress [45,46,50]. Cao et al. [45] suggested that Si improves the light energy distribution between PSI and PSII in plants subjected to drought stress. Exogenous application of Si also increased the tolerance to water deficiency in maize (Zea mays L.) by enhanced photosynthetic efficiency, stomatal conductance, and cell membrane integrity [51].
Silicon treatment increases maize and cantaloupe (Cucumis melo var. reticulatus L. Naud. cv. Galia) tolerance to drought by increased growth and numbers of leaves per plant [51,52]. Fruit length, diameter, flesh thickness, and overall fruit yield were also significantly higher when Si was applied to cantaloupe during drought [52]. For mango, addition of Si increased relative growth rate, net assimilation rate, and relative water content, compared to the plants under water stress and the control plants [6]. Similarly, Si addition to water stressed Kentucky bluegrass, increased the net photosynthesis, leaf water contents and turf quality [49]. Zhang et al. [48] and Alzahrani et al. [46] reported that Si improved plant growth under water stress in tomato and wheat. On the contrary, Thorne et al. [53] indicated that Si does not significantly affect growth during drought stress in wheat.
Silicon also increases the water potential and the relative water content in leaves of sugarcane and wheat during drought stress [44,46]. Additionally, Si increases the content of chlorophylls and carotenoids under water stress, compared with control tomato and mango plants [6,48].
During drought stress, treating plants with Si enhances the plant Si concentrations in high and low Si-accumulating wheat cultivars [53]. If present at high levels in the soil, Si can be accumulated in the above-ground plant tissues even in nonaccumulator species (i.e., oilseed rape [Brassica napus L.]), and can improve water uptake under drought stress [54]. Foliar application of Si decreases the accumulation of Ni 2+ , Cd 2+ and Cr 3+ in maize leaves and grain if the plants are under drought stress by metal-contaminated irrigation water [51].
Si might also trigger the transcription of genes that are related to antioxidant defense, photosynthesis, osmotic adjustment, lignin, and suberin metabolism, although those connections are yet to be confirmed [55].

Ultraviolet Radiation
The sun is a source of energy for plants. Photosynthetic plants convert this light energy into chemical energy through photosynthesis, for wavelengths between 400 nm and 700 nm. This range thus represents the photosynthetically active radiation [56]. The most efficient visible light for photosynthesis is within the range of the best absorption for chlorophyll a and chlorophyll b. These wavelengths are in the blue (450-500 nm) and red (610-760 nm) spectra [57,58]. Wavelengths from 100 nm to 400 nm are known as ultraviolet (UV) radiation, and are divided into three ranges: UV-A, UV-B, and UV-C. UV-C radiation (100-280 nm) is very harmful, but it is entirely absorbed by the atmosphere. UV-B radiation (280-315 nm) is mainly absorbed in the ozone layer, but the amount of radiation that reaches the Earth's surface can be a stress factor for plants [59]. UV-A radiation (315-400 nm) is not absorbed in the ozone layer and is also the least dangerous of the UV ranges for organisms, due to its lower energy [58,59].

Plant Damage by UV-B Radiation
UV-B radiation is essential for plant growth and metabolism. However, high UV-B exposure can cause damage to plant cells, which is then reflected in the physiological processes of the whole plant. Generally, dicotyledons are more sensitive to UV-B radiation than monocotyledons [60]. UV-B can induce DNA damage and mutations in DNA replication [61,62]. Staxén et al. [63] reported that UV-B radiation affects the plant cell cytoskeleton by causing breaks in cortical microtubules, and in some cases, this can inhibit cell division. UV-B also degrades amino acids, which leads to inactivation of proteins and enzymes [64]. Damage to lipids in plant cell membranes is also caused by UV-B radiation in the presence of oxygen, which is known as lipid peroxidation [1]. Lipid peroxidation can be defined by measuring the malondialdehyde content, which is formed through oxidation and degradation of polyunsaturated fatty acids [65]. UV-B can destroy pigments in the photosynthetic apparatus, such as chlorophylls and carotenoids, and consequently lower the fluorescence of photosystem (PS)II [66][67][68]. It has been shown that UV-B degrades IAA and can down-regulate genes associated with IAA activity [69,70]. IAA is one of the vital plant hormones of the auxin class, and its shortage results in stem growth reduction [70]. A decrease in the activity of Rubisco (an enzyme essential for carbon fixation) of UV-B-exposed plants leads to lower carbon dioxide fixation and oxygen formation, as was reported by Kataria et al. [60]. UV-B-treated barley, wheat, cotton, sorghum, and amaranth contain less chlorophyll than the untreated plants [60,71]. UV-B radiation can result in smaller specific leaf weights [72]. UV-B can affect the photosynthesis rate also indirectly by reducing stomatal conductance, changing leaf anatomy [73], and increasing leaf thickness [74], which can change the light penetration into the leaf, and thus change the morphology of the canopy [59].

Plant Protection from UV Radiation
Plants have evolutionary developed morphological structures to prevent UV radiation from entering into the shoot tissues and physiological mechanisms to avoid and repair the damage caused by UV. The first morphological barrier against UV radiation is the epidermis, with various structures as cuticle and trichomes. Epidermal cells usually contain UV-absorbing compounds such as cinnamoyl esters, flavones, flavonols, and anthocyanins which synthesis increases in the presence of UV-B radiation to prevent damage in photosynthetically active mesophyll [75][76][77]. The trichomes behave as optical filters, screening out wavelengths that could damage sensitive tissues. Protection from strong visible radiation is also afforded by increased surface light reflectance [78].
When penetrating into the cells UV-B rays generate free radicals and ROS, which cause damage to DNA, proteins, and lipids. Protection against surplus UV radiation can result in elimination of free radicals through enhanced antioxidative enzyme activity that protects the cell and its vital processes and DNA repair mechanisms [79,80]. The enzymatic system that alleviates the negative effects of ROS includes superoxide dismutase, catalase, glutathione reductase, peroxidase, and ascorbate acid peroxidase [79,80]. Superoxide radicals can be transformed into hydrogen peroxide by superoxide dismutase, and later into water by catalase, glutathione peroxidases and peroxiredoxins [81]. The non-enzymatic system includes ascorbic acid, glutathione, carotenoids, proline, polyamines, and phenols [80,82].

UV-B and Silicon
Not many studies on the relationships between UV-B stress and mitigation of stress effects by Si supplementation can be found in the literature. Studies regarding the addition of Si or Si nanoparticles have been reported for crop plants such as rice, wheat, and soybean (Glycine max L.) ( Table 2). Such studies have shown that Si can reduce the negative effects of UV radiation through increased antioxidant capacity [3,4] and lower concentrations of ROS [5].
Availability, and accessibility, of Si can mitigate the harmful effects of UV-B radiation on plants. It has been shown that addition of Si in the form of potassium silicate (K 2 SiO 3 ) mitigates UV-B damage to wheat seedlings (Triticum aestivum L.) through an increase in plant antioxidant compounds and Si levels in leaves [3]. Si treatment increased total plant biomass and contents of chlorophylls (a + b), soluble sugars, anthocyanins, and flavonoids, and reduced superoxide radical (O 2− ) production and malondialdehyde content (which indicates lipid peroxidation) in the wheat seedlings [3].
Mihaličová Malčovská et al. [5] exposed maize seedlings to short-term UV-B radiation. The seedlings were grown hydroponically and treated with Si or left untreated, as the control. In control plants, after UV-B exposure, the content of ROS and thiobarbituric acid reactive substances increased, along with a small increase in the content of total phenols and flavonoids. After UV-B exposure, the maize treated with Si showed only a small increase in flavonoid content, which indicated stress mitigation by Si. Schaller et al. [83] reported that an excess of Si decreased phenol content in leaves and increased its content in the culm. This suggests that phenols and Si have the same functionality in UV protection.
In UV-B-exposed wheat, high levels of superoxide radical and H 2 O 2 have been reported; therefore, lipid peroxidation and electrolyte leakage were increased [84]. Shen et al. [85] reported that treating UV-B stressed soybean with Si resulted in decreased levels of catalase, superoxide dismutase and peroxidase activities. Tripathi et al. [84] also showed differences in the anatomical properties of the leaves. UV-B radiation reduced leaf thickness, size of mesophyll cells, and lignification in the metaxylem vessels. UV-B-exposed wheat showed damage to chloroplasts. Addition of Si and Si nanoparticles diminished the damaging impact of UV-B on the leaves. Moreover, the addition of Si and Si nanoparticles improved lignification and suberization of bundle sheath cells and metaxylem vessels [84].
Shen et al. [86] simulated UV-B radiation at 30% stratospheric ozone depletion on soybean and showed that UV-B-treated plants gain less biomass than the controls. UV-Btreated plants also contained more leaf N and P, and less leaf Mg and Ca. UV-B increased the allocation of P, K, and Ca to the roots. In these soybeans, the addition of Si increased the uptake of P and Mg and favored the allocation of P and Ca to the roots.

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simultaneously, so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H 2 O 2 concentrations, compared to the nontreated plants [4].
Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate.
As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities. Table 3. Effects of silicon supplementation on the measured parameters in the presence of UV-B, drought and their combination as revised from the literature. Arrows indicate increases (↑) or decreases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B × drought) in silicon treated plants.

UV-B UV-B × Drought Reference
Elemental composition Nutrient uptake so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4]. Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate.
As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities. Table 3. Effects of silicon supplementation on the measured parameters in the presence of UV-B, drought and their combination as revised from the literature. Arrows indicate increases (↑) or decreases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B × drought) in silicon treated plants.

UV-B UV-B × Drought Reference
Elemental composition Nutrient uptake [43] P, Mg uptake [86] Leaf Si concentration [3] Leaf C:N ratio [49] Antioxidant capacity [3] Antioxidants Non-enzymatic system Flavonoids [3](↑), [5](↓) Anthocyanin [3] Ascorbic acid [46] Glutathione [46] Proline [44,46,47]  Drought and high UV-B radiation in natural ecosystems often occur simu so knowing how their joint action affects plants is essential. The combinatio drought, and Si addition is not well known, with only a few studies carried o Shen et al. [4] investigated the effects of Si on soybean seedlings under drough B radiation. Both of these stress factors caused membrane damage by lipid pe and osmolyte leakage, which were reduced with Si application. Under UV-B an stresses, Si also decreased the activity of superoxide dismutase and catalase, an free proline levels and H2O2 concentrations, compared to the nontreated plants Grašič et al. [88] reported that the combination of UV and drought affects trations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV rad water shortage had significantly lower Si levels in leaves, compared to plants duced UV radiation, which was connected to lowered transpiration rate. As summarized on Table 3, Si addition to UV-B and drought stress reduces damage, oxidative stress and levels of some antioxidants or antioxidant enzyme Table 3. Effects of silicon supplementation on the measured parameters in the presen drought and their combination as revised from the literature. Arrows indicate increas creases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B in silicon treated plants.

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simu so knowing how their joint action affects plants is essential. The combinatio drought, and Si addition is not well known, with only a few studies carried o Shen et al. [4] investigated the effects of Si on soybean seedlings under drough B radiation. Both of these stress factors caused membrane damage by lipid pe and osmolyte leakage, which were reduced with Si application. Under UV-B an stresses, Si also decreased the activity of superoxide dismutase and catalase, an free proline levels and H2O2 concentrations, compared to the nontreated plants Grašič et al. [88] reported that the combination of UV and drought affects trations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV rad water shortage had significantly lower Si levels in leaves, compared to plants duced UV radiation, which was connected to lowered transpiration rate. As summarized on Table 3, Si addition to UV-B and drought stress reduces damage, oxidative stress and levels of some antioxidants or antioxidant enzyme Table 3. Effects of silicon supplementation on the measured parameters in the presen drought and their combination as revised from the literature. Arrows indicate increas creases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B in silicon treated plants.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simultaneously, so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4].
Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate.
As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities. Table 3. Effects of silicon supplementation on the measured parameters in the presence of UV-B, drought and their combination as revised from the literature. Arrows indicate increases (↑) or decreases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B × drought) in silicon treated plants.

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simu so knowing how their joint action affects plants is essential. The combinatio drought, and Si addition is not well known, with only a few studies carried o Shen et al. [4] investigated the effects of Si on soybean seedlings under drough B radiation. Both of these stress factors caused membrane damage by lipid pe and osmolyte leakage, which were reduced with Si application. Under UV-B an stresses, Si also decreased the activity of superoxide dismutase and catalase, an free proline levels and H2O2 concentrations, compared to the nontreated plants Grašič et al. [88] reported that the combination of UV and drought affects trations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV rad water shortage had significantly lower Si levels in leaves, compared to plants duced UV radiation, which was connected to lowered transpiration rate. As summarized on Table 3, Si addition to UV-B and drought stress reduces damage, oxidative stress and levels of some antioxidants or antioxidant enzyme Table 3. Effects of silicon supplementation on the measured parameters in the presen drought and their combination as revised from the literature. Arrows indicate increas creases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B in silicon treated plants.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simulta so knowing how their joint action affects plants is essential. The combination drought, and Si addition is not well known, with only a few studies carried out Shen et al. [4] investigated the effects of Si on soybean seedlings under drought B radiation. Both of these stress factors caused membrane damage by lipid pero and osmolyte leakage, which were reduced with Si application. Under UV-B and stresses, Si also decreased the activity of superoxide dismutase and catalase, and free proline levels and H2O2 concentrations, compared to the nontreated plants [ Grašič et al. [88] reported that the combination of UV and drought affects S trations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radia water shortage had significantly lower Si levels in leaves, compared to plants u duced UV radiation, which was connected to lowered transpiration rate. As summarized on Table 3, Si addition to UV-B and drought stress reduces m damage, oxidative stress and levels of some antioxidants or antioxidant enzymes a Table 3. Effects of silicon supplementation on the measured parameters in the presence drought and their combination as revised from the literature. Arrows indicate increases creases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B × in silicon treated plants.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simultaneously, so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4].
Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate. As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities. Table 3. Effects of silicon supplementation on the measured parameters in the presence of UV-B, drought and their combination as revised from the literature. Arrows indicate increases (↑) or decreases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B × drought) in silicon treated plants.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simu so knowing how their joint action affects plants is essential. The combinatio drought, and Si addition is not well known, with only a few studies carried o Shen et al. [4] investigated the effects of Si on soybean seedlings under drough B radiation. Both of these stress factors caused membrane damage by lipid pe and osmolyte leakage, which were reduced with Si application. Under UV-B an stresses, Si also decreased the activity of superoxide dismutase and catalase, an free proline levels and H2O2 concentrations, compared to the nontreated plants Grašič et al. [88] reported that the combination of UV and drought affects trations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV rad water shortage had significantly lower Si levels in leaves, compared to plants duced UV radiation, which was connected to lowered transpiration rate. As summarized on Table 3, Si addition to UV-B and drought stress reduces damage, oxidative stress and levels of some antioxidants or antioxidant enzyme Table 3. Effects of silicon supplementation on the measured parameters in the presen drought and their combination as revised from the literature. Arrows indicate increas creases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B in silicon treated plants.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simultaneously, so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4].
Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate. As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities. Table 3. Effects of silicon supplementation on the measured parameters in the presence of UV-B, drought and their combination as revised from the literature. Arrows indicate increases (↑) or decreases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B × drought) in silicon treated plants.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simultaneously, so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4].
Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate. As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simultaneously, so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4].
Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate. As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simultaneously, so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4].
Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate. As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities. Table 3. Effects of silicon supplementation on the measured parameters in the presence of UV-B, drought and their combination as revised from the literature. Arrows indicate increases (↑) or decreases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B × drought) in silicon treated plants.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simultaneously, so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4].
Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate. As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simultaneously, so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4].
Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate. As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simultaneously, so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4].
Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate.
As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simultaneously, so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4].
Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate. As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simu so knowing how their joint action affects plants is essential. The combinatio drought, and Si addition is not well known, with only a few studies carried o Shen et al. [4] investigated the effects of Si on soybean seedlings under drough B radiation. Both of these stress factors caused membrane damage by lipid pe and osmolyte leakage, which were reduced with Si application. Under UV-B an stresses, Si also decreased the activity of superoxide dismutase and catalase, an free proline levels and H2O2 concentrations, compared to the nontreated plants Grašič et al. [88] reported that the combination of UV and drought affects trations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV rad water shortage had significantly lower Si levels in leaves, compared to plants duced UV radiation, which was connected to lowered transpiration rate. As summarized on Table 3, Si addition to UV-B and drought stress reduces damage, oxidative stress and levels of some antioxidants or antioxidant enzyme Table 3. Effects of silicon supplementation on the measured parameters in the presen drought and their combination as revised from the literature. Arrows indicate increas creases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B in silicon treated plants.

Drought
UV-B

Reference
Elemental composition Nutrient uptake [43] P, Mg uptake [86] Leaf Si concentration [3] Leaf C:N ratio [49] Antioxidant capacity [3] Antioxidants Non-enzymatic system Flavonoids [3](↑), [5](↓) Anthocyanin [3] Ascorbic acid [46] Glutathione [46] Proline [44,46,47 trations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate. As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities. Table 3. Effects of silicon supplementation on the measured parameters in the presence of UV-B, drought and their combination as revised from the literature. Arrows indicate increases (↑) or decreases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B × drought) in silicon treated plants.

Drought
UV-B

Reference
Elemental composition Nutrient uptake [43] P, Mg uptake [86] Leaf Si concentration [3] Leaf C:N ratio [49] Antioxidant capacity [3] Antioxidants Non-enzymatic system Flavonoids [3](↑), [5](↓) Anthocyanin [3] Ascorbic acid [46] Glutathione [46] Proline [44,46,47 Gibberellic acid free proline levels and H2O2 concentrations, compared to the nontreated plants [4]. Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate. As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities. Table 3. Effects of silicon supplementation on the measured parameters in the presence of UV-B, drought and their combination as revised from the literature. Arrows indicate increases (↑) or decreases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B × drought) in silicon treated plants.

Drought
UV-B

Reference
Elemental composition Nutrient uptake [43] P, Mg uptake [86] Leaf Si concentration [3] Leaf C:N ratio [49] Antioxidant capacity [3] Antioxidants Non-enzymatic system Flavonoids [3](↑), [5](↓) Anthocyanin [3] Ascorbic acid [46] Glutathione [46] Proline [44,46,47 Cytokinins and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4]. Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate. As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities. Table 3. Effects of silicon supplementation on the measured parameters in the presence of UV-B, drought and their combination as revised from the literature. Arrows indicate increases (↑) or decreases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B × drought) in silicon treated plants.

Drought
UV-B
As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities. Table 3. Effects of silicon supplementation on the measured parameters in the presence of UV-B, drought and their combination as revised from the literature. Arrows indicate increases (↑) or decreases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B × drought) in silicon treated plants.

Drought
UV-B

UV-B × Drought
Reference position Nutrient uptake [43] P, Mg uptake [86] Leaf Si concentration [3] Leaf C:N ratio [49] Antioxidant capacity [3] Non-enzymatic system Flavonoids [3](↑), [5](↓) Anthocyanin [3] Ascorbic acid [46] Glutathione [46] Proline [44,46,47 Oxidative stress Reactive oxygen species and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4]. Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate.
As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities. Table 3. Effects of silicon supplementation on the measured parameters in the presence of UV-B, drought and their combination as revised from the literature. Arrows indicate increases (↑) or decreases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B × drought) in silicon treated plants.

Drought
UV-B
As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities. Table 3. Effects of silicon supplementation on the measured parameters in the presence of UV-B, drought and their combination as revised from the literature. Arrows indicate increases (↑) or decreases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B × drought) in silicon treated plants.

Drought
UV-B

Reference
Elemental composition Nutrient uptake [43] P, Mg uptake [86] Leaf Si concentration [3] Leaf C:N ratio [49] Antioxidant capacity [3] Antioxidants Non-enzymatic system Flavonoids [3](↑), [5](↓) Anthocyanin [3] Ascorbic acid [46] Glutathione [46] Proline [44,46,47 Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4]. Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate.
As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities. Table 3. Effects of silicon supplementation on the measured parameters in the presence of UV-B, drought and their combination as revised from the literature. Arrows indicate increases (↑) or decreases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B × drought) in silicon treated plants.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simultaneously, so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4].
Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate.
As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities. Table 3. Effects of silicon supplementation on the measured parameters in the presence of UV-B, drought and their combination as revised from the literature. Arrows indicate increases (↑) or decreases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B × drought) in silicon treated plants.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simu so knowing how their joint action affects plants is essential. The combinatio drought, and Si addition is not well known, with only a few studies carried o Shen et al. [4] investigated the effects of Si on soybean seedlings under drough B radiation. Both of these stress factors caused membrane damage by lipid pe and osmolyte leakage, which were reduced with Si application. Under UV-B an stresses, Si also decreased the activity of superoxide dismutase and catalase, an free proline levels and H2O2 concentrations, compared to the nontreated plants Grašič et al. [88] reported that the combination of UV and drought affects trations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV rad water shortage had significantly lower Si levels in leaves, compared to plants duced UV radiation, which was connected to lowered transpiration rate.
As summarized on Table 3, Si addition to UV-B and drought stress reduces damage, oxidative stress and levels of some antioxidants or antioxidant enzyme Table 3. Effects of silicon supplementation on the measured parameters in the presen drought and their combination as revised from the literature. Arrows indicate increas creases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B in silicon treated plants.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simultaneously, so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4].
Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate.
As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simu so knowing how their joint action affects plants is essential. The combinatio drought, and Si addition is not well known, with only a few studies carried o Shen et al. [4] investigated the effects of Si on soybean seedlings under drough B radiation. Both of these stress factors caused membrane damage by lipid pe and osmolyte leakage, which were reduced with Si application. Under UV-B an stresses, Si also decreased the activity of superoxide dismutase and catalase, an free proline levels and H2O2 concentrations, compared to the nontreated plants Grašič et al. [88] reported that the combination of UV and drought affects trations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV rad water shortage had significantly lower Si levels in leaves, compared to plants duced UV radiation, which was connected to lowered transpiration rate.
As summarized on Table 3, Si addition to UV-B and drought stress reduces damage, oxidative stress and levels of some antioxidants or antioxidant enzyme Table 3. Effects of silicon supplementation on the measured parameters in the presen drought and their combination as revised from the literature. Arrows indicate increas creases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B in silicon treated plants.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simultaneously, so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4].
Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate.
As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simultaneously, so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4].
Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate.
As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simultaneously, so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4].
Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate.
As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simu so knowing how their joint action affects plants is essential. The combinatio drought, and Si addition is not well known, with only a few studies carried o Shen et al. [4] investigated the effects of Si on soybean seedlings under drough B radiation. Both of these stress factors caused membrane damage by lipid pe and osmolyte leakage, which were reduced with Si application. Under UV-B an stresses, Si also decreased the activity of superoxide dismutase and catalase, an free proline levels and H2O2 concentrations, compared to the nontreated plants Grašič et al. [88] reported that the combination of UV and drought affects trations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV rad water shortage had significantly lower Si levels in leaves, compared to plants duced UV radiation, which was connected to lowered transpiration rate.
As summarized on Table 3, Si addition to UV-B and drought stress reduces damage, oxidative stress and levels of some antioxidants or antioxidant enzyme Table 3. Effects of silicon supplementation on the measured parameters in the presen drought and their combination as revised from the literature. Arrows indicate increas creases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B in silicon treated plants.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simultaneously, so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4].
Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate.
As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities. Table 3. Effects of silicon supplementation on the measured parameters in the presence of UV-B, drought and their combination as revised from the literature. Arrows indicate increases (↑) or decreases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B × drought) in silicon treated plants.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simultaneously, so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4].
Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate.
As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities. Table 3. Effects of silicon supplementation on the measured parameters in the presence of UV-B, drought and their combination as revised from the literature. Arrows indicate increases (↑) or decreases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B × drought) in silicon treated plants.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simultaneously, so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4].
Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate.
As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities. Table 3. Effects of silicon supplementation on the measured parameters in the presence of UV-B, drought and their combination as revised from the literature. Arrows indicate increases (↑) or decreases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B × drought) in silicon treated plants.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simultaneously, so knowing how their joint action affects plants is essential. The combination of UV-B, drought, and Si addition is not well known, with only a few studies carried out to date. Shen et al. [4] investigated the effects of Si on soybean seedlings under drought and UV-B radiation. Both of these stress factors caused membrane damage by lipid peroxidation and osmolyte leakage, which were reduced with Si application. Under UV-B and drought stresses, Si also decreased the activity of superoxide dismutase and catalase, and lowered free proline levels and H2O2 concentrations, compared to the nontreated plants [4].
Grašič et al. [88] reported that the combination of UV and drought affects Si concentrations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV radiation and water shortage had significantly lower Si levels in leaves, compared to plants under reduced UV radiation, which was connected to lowered transpiration rate.
As summarized on Table 3, Si addition to UV-B and drought stress reduces membrane damage, oxidative stress and levels of some antioxidants or antioxidant enzymes activities. Table 3. Effects of silicon supplementation on the measured parameters in the presence of UV-B, drought and their combination as revised from the literature. Arrows indicate increases (↑) or decreases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B × drought) in silicon treated plants.

Drought
UV-B

Silicon, UV-B, and Drought
Drought and high UV-B radiation in natural ecosystems often occur simu so knowing how their joint action affects plants is essential. The combinatio drought, and Si addition is not well known, with only a few studies carried o Shen et al. [4] investigated the effects of Si on soybean seedlings under drough B radiation. Both of these stress factors caused membrane damage by lipid pe and osmolyte leakage, which were reduced with Si application. Under UV-B an stresses, Si also decreased the activity of superoxide dismutase and catalase, an free proline levels and H2O2 concentrations, compared to the nontreated plants Grašič et al. [88] reported that the combination of UV and drought affects trations in leaves. Proso millet (Panicum miliaceum L.) under ambient UV rad water shortage had significantly lower Si levels in leaves, compared to plants duced UV radiation, which was connected to lowered transpiration rate.
As summarized on Table 3, Si addition to UV-B and drought stress reduces damage, oxidative stress and levels of some antioxidants or antioxidant enzyme Table 3. Effects of silicon supplementation on the measured parameters in the presen drought and their combination as revised from the literature. Arrows indicate increas creases (↓) of each parameter under UV-B, drought, or combined stress conditions (UV-B in silicon treated plants.

Drought
UV-B

Conclusions
Silicon is an abundant chemical element that provides limited but important contributions to the normal functionality of plants during stress. Deciphering the mechanisms of how silicon contributes to abiotic stress alleviation is still a great challenge to plant biolo-gists. Based on the literature included here, Si can either increase or decrease the contents of different molecules or parameters in plants under drought and/or UV-B stress conditions. Studies have shown that Si reduces the harmful effects of UV radiation through increased antioxidant capacity, anthocyanin and soluble sugars levels, the uptake of some nutrients, total biomass of plants, cell lignification and suberinization. Si also decreases the content of phenol substances and osmolyte leakage. In relation to drought, Si increases plant growth, the number of leaves, turf quality, root:shoot ratio, expression of photosynthesis-related genes, relative water content, leaf water content, the contents of some plant hormones, the activity of ascorbate peroxidase, and the levels of ascorbic acid, glutathione and carotenoids.
Si treated plants respond similarly to drought or UV-B stress. Addition of Si in plants exposed to increased UV-B radiation and drought increases photosynthesis rate and chlorophyll content, decreases the activity of catalase and superoxide dismutase, and decreases the content of malondialdehyde. Therefore, plants contain less reactive oxygen species.
Knowledge of the combined effects of drought and UV-B radiation is fundamental here, because these usually occur together. The combination of those two stress factors has not been studied in detail yet, but the first studies have showed some mitigating effects of Si.